I know this is somewhat off topic, but from reading posts there seems to be some good expertise in this area. So, here goes. What is it about diesel that allows the engine to generate better gas mileage and greater torque? Is there something inherent in the fuel itself? Otherwise, why couldn't you design a gasoline engine with similar characteristics to a diesel engine? Why do diesel engines tend to last longer? Part of the reason I am asking is I just got a 2008 MB 320 Bluetec that is unbelievable. Cruising 80mph on the highway, with plenty of top end, getting 34mpg. In addition, it feels like this car is quicker to 60 and above than my 5.7L Hemi Jeep Grand Cherokee. Any ideas?

Diesel has a lot more BTU's per gallon. More energy results in more power and thus better mileage.

Diesel engines don't have spark plugs. They get their combustion by greatly compressing the fuel/air mixture until it ignites. This higher compression ratio results in more force when it ignites and that results in high torque.

The high compression means that the engine needs to be built stronger to handle it. So, they're much beefier than gasoline engines. This results in a heavier and longer lasting engine.

You can't compress gasoline like diesel because it would ignite prematurely. That's what happens when your engine knocks. Also, they could make gasoline engines that last much longer, but they don't need to because the rest of the vehicle wears out too.

So, its the nature of the diesel itself that allows for engineering the combustion process differently than gasoline - right?

Also, I have heard that a gallon of diesel fuel takes more raw oil than gasoline but gasoline is much more refined so when the price per barrel of oil goes up, diesel tends to go up more than gasoline, but gasoline rises when refinery capacity is a problem (along with oil prices). Is that true? For example, previously, when gasoline prices rose it was mostly due to a lack of refinery capacity, not oil prices. Back then (several months ago), a gallon of diesel was about the same price as mid-grade gasoline. However, right now, diesel is approx. $.15 per gallon more than premium-grade gasoline, because the price per barrel has gone thru the roof. Is that likewise true?

I'm going to disagree with TuneMan just a bit because he's generally correct. Diesels have more torque primarily because they are designed to operate at low RPM's. Look at a gas farm tractor engine and you'll see that gas engines can have very high torque ratings and long lives when designed to do so. Also the primary reason diesels get better mileage is because they operate without intake vacuum so there are no pumping losses at part throttle operation which effect gas vehicles only. Diesel only has about 11% more btu/gal and with the new low sulfur stuff that will go to about 10%. High RPM, light weight diesels exist in cars like VWs and their gas mileage is pretty proportional when viewed as a ratio of HP. So, there really isn't that much difference in the fuel compared to the design criteria of the engines themselves. All things being equal, which they are not due to consumer demands, gas engines would likely last longer then diesels as the combustion process is easier to keep clean. John...

Engines are sexy....what other machine goes suck, squeeze, bang, and blow. Or in the case of a 2 cycle, squeeze, bang, while constantly being blown.

anyway, a diesel is a compression ignition engine not spark ignited. Tman is pretty spot on, that it's the basic fuel and that allows for a harder blast off of the power stroke, and a beefier engine over all built around that. Performance of a diesel engine comes right out of engine life unfortunately. The more hp per cubic inch the higher the BMEP, or brake mean effective pressure and/or the higher the piston speed, these are the 2 most critical elements in diesel engine life. So a hot rod diesel will not last like a 900rpm big bore that lopes along for a living. Benzos and the aluminum block industrial or marine vw's put a lot of air into the cylinder via turbos, blowers or intercolers and aftercoolers (bonus points if you know the difference)and get lots of hp, but at the cost of engine life, they will still outlast a sparky by a long shot however.

The reason that a diesel delivers more tq @ lower rpms is due to the higher cylinder pressures created by long stroke/hi compression. This is caused by cylinder head and piston design but also cam design as there is not much valve overlap.

Intercooler is the heat exchanger used to cool a charge between superchargers, either mechanical or exhaust turbine driven, on a two or more stage forced induction system. Aftercooler is the heat exchanger after the supercharger and before the combustion chamber. But who cares. There are so few multiple stage forced induction systems in passenger use that the everyone can guess that intercooler means between the turbo and engine in common use. Can I trade in my bonus points for a hockey card?

I could calculate that up but it would take some work. I'm too busy trying to book a Braniff flight. Thanks.

Wakeshoe, on the acceleration you have to know the vehicle weights and average HP to understand why one might go better. Turbo's always feel great because you get that 'kick' when the turbo starts to work. It doesn't make it faster necessarily.

Art, The specs on my 2005 Jeep Grand Cherokee Limited with the 5.7L Hemi are that it weighs something like 4400lbs and the engine is rated at 330HP. It does 0-60 in just under 7 seconds and quarter mile around 15 seconds. The specs on my 2008 MB E320 Bluetec are that it weighs something like 3860lbs and the engine is rated at 210HP. It does 0-60 in 6.6 seconds. I could not find quarter mile times for it but I am certain it is quicker than 15 seconds.

The biggest difference, apart from the 0-60 times is when you are cruising on the highway at 60+mph. The pedal on the MB feels really strong and just pushing it slightly makes the car really move, without dropping into passing gear. The Jeep, on the other hand, when cruising, you push the pedal, which feels soft and you get good get up and go, but it has to drop into passing gear to get moving. The torque of the MB plus the turbo make this a really hot car, yet getting ridiculously great gas mileage. So far, after filling up 4 times, the worst gas mileage I have seen is 31mpg, combined city/highway. My jeep gets 13-15mpg combined city/highway.

Sounds like a great car wakeshoe. That extra getup you feel on the highway is the aero drag that really wakes a difference at speeds over 50. The usable Hp gets closer because the MB has much less drag than the Jeep. Running a 1/4 in under 15 is a very good time for a truck, or anything really.

I purchased a VW Jetta TDI about 6 months ago. I love this thing! I am averaging between 44-46 mpg on every tank and the torque on these little diesels is great. I can't wait for the rest of the auto makers to bring their oil burners accross the pond.

WakeShoe - Are you running any BioD in your MB CDI? I have been running B20 for 15k now and its all I ever use.

We bought our Jetta to serve as basic, economical transportation... a commuter car for a long commute. I wouldn't call it a sports sedan but it definitely helps offset the money spent on fuel between my Dodge Ram 4x4 and Malibu VLX. My wife used to have an SUV that got 17mpg. When you drive 20K to 25K miles per year the money saved on fuel alone nearly makes the payment on the car. She no longer has a long commute but we're still keeping the car. Every now and then she mentions that she wants a BMW 535i or a VW Tourag II TDI but we're quite happy saving the money for now.

I'm with you Chad, we've got a 02 tdi that averages 47 with the best of 52 (at 80 with the a/c on I might add ). Offsets the boat and truck gas usage nicely. Dad's 06 is seeing ~ what you are as far as mpg's.

JBC, You mentioned the MB CDI engine. The E320 used to use that engine until it came out with the Bluetec engine in 2007 (i.e., the E320 CDI ended with 2006). MB is currently only putting the Bluetec engine in the E320 - they use the CDI engine in the ML, GL and R series. As far as biodiesel, I haven't run into any yet. There has been a sign on 35N between Waco and Dallas for what seems like 2 years now that a BioWilly station is in the works, but nothing so far. MB says it only supports B5 biodiesel (5% biodiesel).

I looked at VW's site and I can't see where you can configure the 2008 Jetta with the TDI engine. It seems the only diesel is the V10 twin turbo TDI in the Toureg II with a list price of slightly over $69K. The list on my E320, nicely configured, was $54K. What other diesel cars are available in the US in 2008????

Shoe-VW took the 07 and 08 years off to get the TDI into compliance with the new USLD and T2B5 emissions req's for diesels. They will be back sometime in mid 08 with an 09 model that will both be compliant and get ~60 mpg on the highway. That is an accomplishment because of the size of the new jetta's. Check out the news section of the tdiclub forums.

Diesel has suffered from a very poor advertising and public relations campaign; or lack thereof. Unless they're buying a heavy-duty truck most people won't even consider them. My own sister thought I was nuts too. Even though roughly half of the cars sold in Europe, and yes that means many MB, Audi, VW, and BMW are diesels we Americans as a whole are just ignorant of them and thus there hasn't been a lot of demand.

JBC, I couldn't be happier with my MB. MB does tend to nickle and dime you a bit, but the car is solid. I got the P2 package, split folding rear seats, voice command and the wood steering wheel. Inside the car you cannot tell it is a diesel. Outside the car, when idling, you can barely hear the distinctive clatter of the diesel. According to the dealer, the 2008 E320 is the world's safest car by a large margin based on some magazine study that tracks that stuff. Having driven both the E320 Bluetec and the E350, I don't know why anyone would go with the E350. The E320 Bluetec is quicker and faster than the E350 and gets significantly better fuel mileage. If you don't care about fuel mileage, you might want to try the E550 or the E-series AMG, both are rockets. I think your father would be very pleased with the E320 Bluetec.